Lasers are used in many applications where the oscillation frequency of the device is utilized as a clock. At the extreme end of applications stabilities better than 1 part in 1014 enables lasers to be used to synchronize clocks worldwide for highly precise time measurements. These systems require extremely high degrees of isolation against environmental disturbances (such as temperature variations and vibrations) since even minute changes in the length of laser cavities causes variations in the laser frequency. Stable lasers are also required in applications such as coherent laser radar systems. In these systems laser radiation is sent from a sensor location to a target that may be many km distant and the change in phase of the signal upon return to the sensor is used to measure properties of the target. Such measurements rely on measuring the phase very accurately by heterodyning the return signal with a local oscillator beam and comparing the received phase with the transmitted phase. If there is a change in the local oscillator frequency while the pulse is in transit to the target and back, these phase measurements become inaccurate. For an order of magnitude estimate of stability requirements in these circumstances it is noted that phase errors must typically be <<π radians over the round-trip time t=2 R/c, where R is the target range and c is the speed of light=3·108 m/s. For a target range of 50 km the round-trip time is 0.33 msec giving an angular frequency stability requirement of <<π/0.33 ms, or a frequency stability better than Δf =1.5 kHz. For a laser with an emission wavelength λ=1.5 μm the frequency is given by f=c/λ=2·1014 Hz, thus leading to a fractional frequency stability requirement of Δf/f=7.5 10−12. To further put this into context, the frequency of a laser is determined by a standing wave formed in an optical cavity of length L whose resonant frequency is a multiple m of the quantity c/2L (assuming the cavity is a vacuum). Changing the cavity length by a small amount ΔL causes a frequency deviation magnitude given by |Δf|/f=ΔL/L. For a cavity length of 1 cm, a frequency of 2·1014 Hz, and a frequency stability requirement of 1.5 kHz, the tolerance on the length ΔL is then 7.5·10−14 m, or 0.000075 nm, an extraordinarily small number given that, for example, the diameter of a hydrogen atom is approximately 0.1 nm.
Over the years several techniques have successfully been developed to build lasers with frequency stabilities to meet these stringent demands. This is generally not done by directly stabilizing the laser and its environment, but rather by active means, whereby a highly stable and environmentally isolated “frequency reference” is created such that the laser emission frequency can be stabilized to this frequency reference. Although a number of techniques currently exist the perhaps best known is the so-called PDH technique named after Pound, Drever, and Hall who pioneered the technique for microwave signals and transferred these developments to laser cavities. The invention disclosed herein applies equally well to all techniques that rely on locking a laser to a cavity, such as an etalon. Alternatives to the PDH technique include polarization locking (see for example T. W. Hansch, B. Couillaud, “Laser frequency stabilization by polarization spectroscopy of a reflecting reference cavity,” Opt. Comm., Vol. 35, 3, 441-444, 1980) and tilt locking (see for example B. J. J. Slaggmolen et al. “Frequency Stability of Spatial Mode Interference (Tilt) Locking”, IEEE Journal of Quantum Electronics, vol. 38, no. 11, November 2002). The PDH technique is often favored because it is very robust in many ways, such as being stable against intensity fluctuations of the laser. However the PDH technique and other techniques like it suffer from a significant limitation. These techniques are generally aimed at stabilizing the laser frequency to a reference determined by a passive device, such as an etalon, rather than stabilizing the frequency in absolute terms. Thus, if the reference frequency drifts, for example if the etalon resonance frequency drifts, then the laser frequency drifts along with the reference.
This issue of absolute frequency stability is important in a number of applications and has been addressed in various ways to meet specific requirements. It is obvious that when lasers are used as length standards, the absolute frequency of the laser must be known to extremely high precision. The way this is normally achieved is to stabilize the frequency against an atomic resonance frequency. Since such resonance frequencies are very specific, stabilizing a laser at an arbitrary predetermined frequency against such a reference requires complex frequency conversion stages. At a lower precision level frequencies must also be absolutely known for optical communications systems. Fiber optics systems are commonly operated on the so-called ITU (International Telecommunications Union) grid that defines absolute frequency channels to be used by the communications devices. Locking of frequencies to the ITU channels is therefore required. However, the current ITU grid is defined based on frequency channels separated by 50-100 GHz, so it is generally sufficient for communications lasers to be stabilized to on the order of 1 GHz. This relatively relaxed condition can typically be met using wavelength locking techniques that, for example, utilize a temperature stabilized low finesse etalon and a tunable laser to ensure that light is transmitted through the etalon. A third example where absolute frequency stability is required is when coherent laser radar systems are simultaneously used in multiple locations. In such circumstances, if a phase measurement made in one place is to be correlated with one made in a different location, the “clock” lasers used to establish validity of the interferometric phase measurements must operate synchronously. To ensure such clock synchronization the simple wavelength locking techniques used in telecommunications systems are highly unsuitable because sufficient stabilities cannot be guaranteed with simple etalon transmission techniques. On the other hand the complex frequency conversion schemes used for length standards are also unsuited because of complexity, cost, and bulk.
One system has been proposed by S. Sandford and C. Antill, Jr. (“Laser frequency control using an optical resonator locked to an electronic oscillator”, IEEE Journal of Quantum Electronics, vol. 33, pp. 1991, Nov. 1997) to stabilize the cavity free spectral range (FSR) frequency, in order to obtain absolute frequency stability. In the Sandford and Antill method two laser systems are locked to two adjacent etalon cavity modes, and their difference frequency, located in the radio frequency (RF) domain, is then stabilized by means of phase/frequency comparison to a stable RF reference. Any drift of the etalon dimension is detected as a change of FSR, and hence, permits corrective action to take place that holds the FSR constant. If the FSR isn't changing, then the optical frequencies themselves are constrained. One obvious drawback with this approach is the need for two lasers, which results in additional cost and system complexity. Additionally the FSR in this method is being measured as a difference between two independent laser locks. This causes locking noise to enter into the FSR measurements at a level of the square root of 2 times the locking noise strength assuming equivalent locks. It would be preferable to have a method to measure FSR that had no sensitivity to laser locking noise.